This patent relates to an improved process for manufacturing a toothbrush. More specifically, the patent relates to a process for manufacturing hollow handled toothbrushes. The process utilizes a gas which is injected from a specific location in a toothbrush mold to assist in blowing a molten plastic therethrough. The injection of the gas in a specific location results in a more efficient process for manufacturing hollow handled toothbrushes.
Toothbrushes are typically manufactured using an injection molding process. The injection molding process is characterized by providing a mold in the shape of the toothbrush and injecting molten plastic through a hot channel nozzle into the mold. The toothbrush is then cooled and ejected from the mold.
Toothbrushes may be difficult to handle for several reasons. Children may have difficulty handling toothbrushes due to the size of the child""s hands. People with arthritis also sometimes have difficulty handling toothbrushes, due to difficulty in flexing the joints in their hands. Handicapped people may also have difficulty handling toothbrushes. There has been a recent trend to provide toothbrushes with relatively large cross section handles to make handling the toothbrush easier. Additionally, the larger cross section handles on the toothbrushes may be better for the user from an ergonomics point of view.
The manufacture of toothbrushes with larger cross section handles has several drawbacks. Firstly, the toothbrush is more expensive due to the use of more plastic to make the toothbrush. Secondly, the cost of manufacture is increased because the time to cool the toothbrush increases. The increase in cooling time is due to the increased amount of hot plastic and the larger cross section of the toothbrush. Accordingly, there is a need for a more efficient process for making toothbrushes with larger cross section handles.
European Patent No. EP 668140 discloses the use of air assist technology to make toothbrushes with large cross section handles. In the process, molten plastic is injected near the base of the toothbrush handle, perpendicular to the lengthwise direction of the toothbrush mold to partially fill the toothbrush mold. A hot needle is then inserted into the molten plastic, also near the base of the toothbrush handle and also perpendicular to the lengthwise direction of the toothbrush mold. A gas is blown through the needle to press the molten plastic against the walls of the toothbrush mold. The disclosed process may be modified to inject a separate, less expensive polymer in place of the gas, in which case there is produced a solid handle in which the interior portions are made from said separate polymer. In either case, the process is completed by injecting a small amount of plastic to close the hole left by the needle.
European Patent No. EP 721832 also discloses the use of air assist technology to make toothbrushes with large cross section handles. In the disclosed process, molten plastic is injected near the head of the toothbrush mold, perpendicular to the lengthwise direction of the toothbrush mold to partially fill the toothbrush mold. A hot needle is then inserted into the molten plastic, also near the head of the toothbrush, and also perpendicular to the lengthwise direction of the toothbrush mold. A gas is then blown through the needle to press the molten plastic against the walls of the toothbrush mold. The process includes transferring the hollow handled brush to a second mold and injecting a separate plastic to fill the toothbrush handle.
We have found that gas injected just above the base of the toothbrush handle, perpendicular to the lengthwise direction of the toothbrush mold, as taught in European Patent No. EP 668140, may result in inadequate hollowing of the toothbrush handle, i.e., some portions of the previously injected polymer remain as a xe2x80x9cmoundxe2x80x9d near the gas injection point and are not uniformly distributed against the walls of the mold. Additionally, it may be difficult to control the flow of the gas sufficiently to prevent hollowing of the toothbrush neck. Depending on the inherent strength of the polymer being used, it may be desirable to avoid formation of neck portions which are hollow.
We have also found that the gas injected near the neck of the toothbrush, perpendicular to the lengthwise direction of the toothbrush mold, as taught in European Patent No. EP 721832, may result in inadequate hollowing of the toothbrush handle. Additionally, the process is likely to lead to a hollowing of the toothbrush neck.
Therefore, despite the disclosure of the references, there is a continuing need for a more efficient process for making toothbrushes with large cross section handles.
The present invention provides a process for manufacturing a toothbrush including: a) providing a toothbrush mold having a head portion, a base portion, a toothbrush cavity located between said head portion and said base portion and having walls, an injection port for injecting molten plastic, and a gas injection port for injecting a gas, wherein the gas injection port is positioned in the end of the base portion of the mold so that gas is injected into the mold cavity substantially centrally thereof and in a direction parallel to the longitudinal axis of the mold; b) injecting a predetermined amount of a molten plastic into the cavity to partially fill the mold cavity; and c) injecting a gas (or a mixture of gases) through the gas injection port to direct, or force, the molten plastic into contact with the walls of the mold cavity, thereby forming a toothbrush having at least some portions which are hollow.